1. Field of the Invention
This invention relates to devices which absorb noise. More particularly, the invention relates to an active sound absorbing system with tunable resonators.
2. Prior Art
In some applications in the field of acoustics devices are needed which reflect or absorb acoustical waves in a specified way. Often these devices should not reflect any acoustical waves and absorb the acoustic energie.
At high frequencies this specified behaviour, e.g. no reflection, can be achieved by simple, passive constructive means, i.e. the of use absorptive materials like foam rubber or glass wool, and by giving the non-reflecting surface a special shape. However at low frequencies the dimensions of absorptive structures get large and impractical.
Active devices which absorb sound are described in e.g. the patents DE4027511 (Mechel),. U.S. Pat. No. 5,812,686 (Hobelsberger), U.S. Pat. No. 5,498,127 (Kraft), which all describe variations of devices with actively simulated acoustic impedances. A more general description of sound absorption can be found in the book xe2x80x9cThe Active Control of Soundxe2x80x9d (P. A. Nelson and S. J. Elliot, Academic Press).
The above mentioned devices may be called devices for simulation of acoustical impedances. The working principle of these devices is the following: At the membrane of an electroacoustical transducer the air pressure is measured by measuring means, e.g. pressure sensors. Based on this measured values the transducer""s membrane is moved by the transducer""s driving means with a certain speed. This speed is calculated based on the measured pressure according to a desired impedance, function. This impedance function describes the dependancy of the speed on the pressure and time, including the momentary pressure value and the pressure""s historical values. Usually this relation will be described as a system of differential equations in the time domain, or as a transfer function in the frequency domain (the resulting Laplace transformated function).
A different approach for an active absorber is described in the patent U.S. Pat. No. 5,119,427 (Hersh et al.). This absorber is constructed as tunable Helmholtz absorber. It is tuned by feeding acoustical waves of appropriate frequency into the resonator chamber. The pressure at the membrane or within the chamber is not measured. To determine the appropriate frequency, phase and amplitude of the acoustical waves to tune the resonator a microphone is placed outside of the chamber which measures the reflections.
One problem at the above mentioned active acoustic absorbers (i.e. patents of Mechel, Kraft, Hobelsberger) is to build the suitable electroacoustic transducer. In principle the membrane at these absorbers should behave like a layer of air: Even small forces should cause large excursions. At low frequencies an electrodynamic transducer, e.g. a moving coil transducer, is well suited. At low frequencies the membrane""s acceleration values are somehow modest, and a large membrane may be used, even at large excursions.
However at higher frequencies the acceleration forces rise to prohibitive values, especially when considerable sound pressure values must be handled. Cone breakups of the transducer would cause disturbances. Additionally the membrane will wear out soon. Summarizing it can be said that a suitable, high excursion transducer for higher frequencies and higher sound pressures would be technically quite demanding and expensive.
In the above metioned patent specification U.S. Pat. No. 5,498,127 (Kraft) the use of e.g. piezoelectric transducers is proposed. However it is well known in the art that the excursion capabilities of piezo-transducers are very limited. So practically the piezo-transducers can only be used at low power applications in the higher frequency range. On the other hand piezo-transducers are simple, light devices with only a few moving parts. And they can handle high pressure waves. They would be favourable transducers for active absorbers.
It is one object of this invention to provide means which allow to use low-excursion transducers, e.g. piezoxe2x80x94transducers, even in high power, middle (e.g. 1 kHz) frequency applications of active absorbers over a broader frequency band.
An other object of the invention is to avoid the necessity to arrange microphons or other sensing means outside of the protected absorber chamber. This arrangement outside of the absorber itself is necessary at all so-called noise cancellation systems and at the device of the above mentioned patent U.S. Pat. No. 5,119,427 (Hersh et al.). It can be imagined that it is sometimes quite difficult to place sensors directly into machine ducts, e.g. within a turbine duct.
One important use of these devices is at aircraft turbines to reduce the emitted fan blade noise by absorbing the fan blade noise at the turbine""s nacelle.
The basic feature of the invented devices is the application of acoustic transformers which are acoustically coupled to the internal transducer of the active sound absorber, i.e. of the device for simulation of an acoustical impedance. These acoustical transformers are shaped to transform the speed and pressure values of acoustical waves at the mouth or orifice of the acoustic transformer into lower speed, higher pressure values at the transducer""s membrane within the absorber. So in fact the transformer is shaped to transform a preferable, higher acoustical impedance at the membrane of the device for simulation of an acoustical impedance into a lower acoustical impedance at the mouth of the transformer. These transformers allow to use relatively high simulated acoustical impedances by transformation of the high impedances into considerably lower impedances at the orifice. Or, more specific, the transformers allow lower speed values of the membrane of the acoustical transducer. In a further embodiment the transformer bundles the low excursion movement of a large membrane into a high excursion movement at the mouth of the acoustical transformer. An additional basic feature is that pressure sensing means are arranged within the acoustical transformer.
Another basic feature is that the absorber""s absorption frequencies are tunable or adaptable during operation without the need for mechanical adaption due to the electronically adaptable simulated acoustical impedance.
Three basic types of acoustic transformers are known: The transmission line type transformer, the Helmholz resonator, and the acoustical horn. These three types can be further combined with each other. The acoustic transformer is used in the invented sound absorbers in combination with low excursion transducers, e.g. a piezo-electric transducer, to transform the low speed, high pressure waves at the surface of the piezo-transducer into lower pressure, higher speed waves at the mouth of the acoustic transformer. Of course other types of transducers (e.g. electrodynamic, electrostatic, magnetodynamic types) may be used too.
As described in the above mentioned patents (Mechel, Hobelsberger) the acoustical impedance (i.e. the relation between local pressure and flow velocity of the gas) at the transducer""s membrane of the device for simulation of an acoustical impedance is actively controlled as follows:
Pressure sensing means, e.g. pressure sensors, are mounted within the acoustical transformer at, that means on or close to, the surface of the transducer""s membrane to measure the air pressure at this location. xe2x80x9cClosexe2x80x9d means in this context acoustically close, that is usually less than {fraction (1/10)} of the shortest acoustical wavelengths at which the device should work. The output signal of the pressure sensing means is conveyed to a calculator or model (both digital or analog or mixed), which delivers its output value to a controller. The controller controls via a power amplifier the movement, i.e. the speed, acceleration, or position and its derivatives, of the transducer""s membrane. The controller forces the membrane to move in reaction to the measured air pressure at the membrane""s surface with a speed according to the desired impedance function. This impedance function is used by the calculator or model to calculate input signals for the controller, e.g. the set point value for speed.
Models and calculators can be of the analog type, or of the digital type (e.g. using digital signal processors, DSP) with analog/digital converters and digital/analog converters as usually used for sound processing purposes. For superior speed control speed sensing means, i.e. a speed sensor, an accelerometer or a position measurement device, must be included to measure the speed of the membrane and gain information about the motional state of the membrane. The controller uses these measured speed values to effectively control the membrane""s speed. The control scheme may be of the single channel type, or multivariable, multildop control schemes may be used (e.g. multivariable state-space controllers). In most cases this closed-loop speed control will be neccessary because cheap, light transducers with nonlinear, nonconstant behaviour and low damping will be used.
For a fuller understanding of the nature of the invention, reference should be made to the following detailed description of the preferred embodiments of the invention, considered together with the accompanying drawings.